A current Long Term Evolution (LTE) system includes two types of frame structures. A frame structure type 1 (as shown in FIG. 1) is applied to a frequency division duplex (Frequency Division Duplexing, FDD) LTE system, and may be referred to as an “FDD frame structure.” A frame structure type 2 (as shown in FIG. 2) is applied to a time division duplex (Time Division Duplexing, TDD) LTE system, and may be referred to as a “TDD frame structure.”
Each subframe (Subframe) in the frame structure type 1 and that in the frame structure type 2 both have a length of 1 ms. Currently, as shown in table 1, seven uplink-downlink configurations exist in the TDD LTE system.
TABLE 1Current uplink-downlink configurations in the TDD LTE systemDownlink-to-uplinkUplink-switch-point periodicity downlink(Downlink-to-UplinkSubframe numberconfigurationSwitch-point periodicity)01234567890 5 msDSUUUDSUUU1 5 msDSUUDDSUUD2 5 msDSUDDDSUDD310 msDSUUUDDDDD410 msDSUUDDDDDD510 msDSUDDDDDDD6 5 msDSUUUDSUUD
In an LTE system, to support hybrid automatic repeat, user equipment (UE) needs to feed back, to a base station by using a physical uplink control channel (PUCCH) and a physical uplink shared channel (PUSCH), a hybrid automatic repeat request-acknowledgement (HARQ-ACK) of a physical downlink shared channel transmission. The hybrid automatic repeat request-acknowledgement may also be referred to as an ACK (Acknowledgment, acknowledgment)/a NACK (Negative Acknowledgement, negative acknowledgement) for short. The UE needs to use a physical hybrid automatic repeat request indicator channel (Physical Hybrid-ARQ Indicator Channel, PHICH) to receive a HARQ-ACK corresponding to the PUSCH.
Currently, for the FDD LTE system, a HARQ-ACK corresponding to a PDSCH transmitted in a downlink subframe n−4 is fed back in an uplink subframe n. For the TDD LTE system, a HARQ-ACK corresponding to a PDSCH transmitted in a downlink subframe n−k is fed back in an uplink subframe n. Herein, k belongs to a set K. A definition of K for each TDD uplink-downlink configuration is shown in table 2.
TABLE 2TDD system downlink association set K: {k0, k1, L kM-1}Uplink-downlinkSubframe nconfiguration01234567890——6—4——7—41——7, 64———7, 64—2——8, 7, 4, 6————8, 7, 4, 6 ——3——7, 6, 116, 55, 4—————4——12, 8, 7, 116, 5, 4, 7——————5——13, 12, 9, 8, 7, 5, 4, 11, 6 ———————6——775——77—
For the TDD LTE system, a HARQ-ACK can be fed back in an uplink subframe. Table 2 may illustrate that any value of k is greater than or equal to 4, that is, a HARQ-ACK can be fed back after at least four subframes. As a result, a HARQ round trip time (RTT) is relatively long, and a data transmission delay is relatively long.
In conclusion, in the current TDD LTE system, uplink control information such as a HARQ-ACK is not fed back in a timely manner, causing a long data transmission delay. Consequently, the TDD LTE system cannot effectively provide a low delay service.